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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="sysadmin-netconfig"></a>Chapter 22. Networking configuration</h2></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="sect1"><a href="netconfig.html#sysadmin-netconfig-hardware">22.1. Hardware</a></span></dt>
<dt><span class="sect1"><a href="netconfig.html#idm961634020">22.2. Configuration of interfaces</a></span></dt>
<dt><span class="sect1"><a href="netconfig.html#idm961628108">22.3. Configuration of interfaces (IPv6)</a></span></dt>
<dt><span class="sect1"><a href="netconfig.html#idm961615700">22.4. Wireless interfaces</a></span></dt>
<dt><span class="sect1"><a href="netconfig.html#idm961615636">22.5. Resolving</a></span></dt>
<dt><span class="sect1"><a href="netconfig.html#idm961606724">22.6. IPv4 Forwarding</a></span></dt>
</dl>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sysadmin-netconfig-hardware"></a>22.1. Hardware</h2></div></div></div>
<h3>
<a name="idm961637004"></a>Network cards (NICs)</h3>
<p>
Drivers for NICs are installed as kernel modules. The module for
your NIC has to be loaded during the initialization of Slackware Linux.
On most systems the NIC is automatically detected and configured
during the installation of Slackware Linux. You can reconfigure
your NIC with the <span class="command"><strong>netconfig</strong></span> command.
<span class="command"><strong>netconfig</strong></span> adds the driver (module) for the detected
card to <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.netdevice</code></html:span>. 
</p>
<p>
It is also possible to manually configure which modules should be
loaded during the initialization of the system. This can be done
by adding a <span class="command"><strong>modprobe</strong></span> line to
<html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.modules</code></html:span>. For example, if you want
to load the module for 3Com 59x NICs (3c59x.o), add the following
line to <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.modules</code></html:span> 
</p>
<pre class="screen">
/sbin/modprobe 3c59x
</pre>
<h3>
<a name="idm961634684"></a>PCMCIA cards</h3>
<p>
Supported PCMCIA cards are detected automatically by the PCMCIA
software. The pcmcia-cs packages from the <span class="quote">“<span class="quote">a</span>”</span> disk set provides
PCMCIA functionality for Slackware Linux.
</p>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="idm961634020"></a>22.2. Configuration of interfaces</h2></div></div></div>
<p>
Network cards are available under Linux through so-called <span class="quote">“<span class="quote">interfaces</span>”</span>.
The <span class="command"><strong>ifconfig</strong></span> command can be used to display the
available interfaces:
</p>
<pre class="screen">
# <span class="command"><strong>ifconfig -a</strong></span>
eth0      Link encap:Ethernet  HWaddr 00:20:AF:F6:D4:AD  
          inet addr:192.168.1.1  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1301 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1529 errors:0 dropped:0 overruns:0 carrier:0
          collisions:1 txqueuelen:100 
          RX bytes:472116 (461.0 Kb)  TX bytes:280355 (273.7 Kb)
          Interrupt:10 Base address:0xdc00 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:77 errors:0 dropped:0 overruns:0 frame:0
          TX packets:77 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:8482 (8.2 Kb)  TX bytes:8482 (8.2 Kb)
</pre>
<p>
Network cards get the name ethn, in which n is a number, starting with
0. In the example above, the first network card (eth0) already has an
IP address. But unconfigured interfaces have no IP address, the
<span class="command"><strong>ifconfig</strong></span> will not show IP addresses for unconfigured
interfaces. Interfaces can be configured in the 
<html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.inet1.conf</code></html:span> file. You can simply read
the comments, and fill in the required information. For example:
</p>
<pre class="screen">
# Config information for eth0:
IPADDR[0]="192.168.1.1"
NETMASK[0]="255.255.255.0"
USE_DHCP[0]=""
DHCP_HOSTNAME[0]=""
</pre>
<p>
In this example the IP address 192.168.1.1 with the 255.255.255.0
netmask is assigned to the first ethernet interface (eth0). If you
are using a DHCP server you can change the <span class="emphasis"><em>USE_DHCP=""</em></span> 
line to <span class="emphasis"><em>USE_DHP[n]="yes"</em></span> (swap <span class="quote">“<span class="quote">n</span>”</span> with 
the interface number). Other variables, except <span class="emphasis"><em>DHCP_HOSTNAME</em></span>
are ignored when using DHCP. For example:
</p>
<pre class="screen">
IPADDR[1]=""
NETMASK[1]=""
USE_DHCP[1]="yes"
DHCP_HOSTNAME[1]=""
</pre>
<p>
The same applies to other interfaces. You can activate the settings
by rebooting the system or by executing <span class="command"><strong>/etc/rc.d/rc.inet1</strong></span>.
It is also possible to reconfigure only one interface with
<span class="command"><strong>/etc/rc.d/rc.inet1 ethX_restart</strong></span>, in which
<span class="emphasis"><em>ethX</em></span> should be replaced by the name of the interface
that you would like to reconfigure.
</p>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="idm961628108"></a>22.3. Configuration of interfaces (IPv6)</h2></div></div></div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="idm961627876"></a>22.3.1. Introduction</h3></div></div></div>
<p>
<acronym class="acronym">IPv6</acronym> is the next generation internet protocol. One
of  the advantages is that it has a much larger address space. In IPv4
(the internet protocol that is commonly used today) addresses are
32-bit, this address space is almost completely used right now, and
there is a lack of IPv4 addresses.  IPv6 uses 128-bit addresses, which
provides an unimaginable huge address space (2^128 addresses). IPv6
uses another address notation, first of all hex numbers are used
instead of decimal numbers, and the address is noted in pairs of
16-bits, separated by a colon (<span class="quote">“<span class="quote">:</span>”</span>).  Let's have a look
at an example address: </p>
<pre class="screen">
fec0:ffff:a300:2312:0:0:0:1
</pre>
<p>
A block of zeroes can be replaced by two colons (<span class="quote">“<span class="quote">::</span>”</span>). Thus,
the address above can be written as:
</p>
<pre class="screen">
fec0:ffff:a300:2312::1
</pre>
<p>
Each IPv6 address has a prefix. Normally this consists of two elements:
32 bits identifying the address space the provider provides you, and a
16-bit number that specifies the network. These two elements form the
prefix, and in this case the prefix length is 32 + 16 = 48 bits. Thus,
if you have a /48 prefix you can make 2^16 subnets and have 2^80 hosts
on each subnet. The image below shows the structure of an IPv6 address
with a 48-bit prefix.
</p>
<div class="figure">
<a name="ipv6address"></a><p class="title"><b>Figure 22.1. The anatomy of an IPv6 address</b></p>
<div class="figure-contents"><div class="mediaobject"><img src="../../images/ipv6addr.png" alt="The anatomy of an IPv6 address"></div></div>
</div>
<br class="figure-break"><p>
There are a some specially reserved prefixes, most notable include:
</p>
<div class="table">
<a name="idm961623780"></a><p class="title"><b>Table 22.1. Important IPv6 Prefixes</b></p>
<div class="table-contents"><table summary="Important IPv6 Prefixes" border="1">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>Prefix</th>
<th>Description</th>
</tr></thead>
<tbody>
<tr>
<td>fe80::</td>
<td>Link local addresses, which are not routed.</td>
</tr>
<tr>
<td>fec0::</td>
<td>Site local addresses, which are locally routed, but not on or to the internet.</td>
</tr>
<tr>
<td>2002::</td>
<td>6to4 addresses, which are used for the transition from IPv4 to IPv6.</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break">
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="idm961621684"></a>22.3.2. Slackware Linux IPv6 support</h3></div></div></div>
<p>
The Linux kernel binaries included in Slackware Linux do not support IPv6
by default, but support is included as a kernel module. This module can be
loaded using <span class="command"><strong>modprobe</strong></span>:
</p>
<pre class="screen">
# <span class="command"><strong>modprobe ipv6</strong></span>
</pre>
<p>
You can verify if IPv6 support is loaded correctly by looking at the 
kernel output using the <span class="command"><strong>dmesg</strong></span>:
</p>
<pre class="screen">
$ <span class="command"><strong>dmesg</strong></span>
[..]
IPv6 v0.8 for NET4.0
</pre>
<p>
IPv6 support can be enabled permanently by adding the following line to
<html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.modules</code></html:span>:
</p>
<pre class="screen">
/sbin/modprobe ipv6
</pre>
<p>
Interfaces can be configured using <span class="command"><strong>ifconfig</strong></span>. But it
is recommended to make IPv6 settings using the <span class="command"><strong>ip</strong></span>
command, which is part of the <span class="quote">“<span class="quote">iputils</span>”</span> package that can
be found in the <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">extra/</code></html:span> directory of the Slackware Linux 
tree.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="idm961618052"></a>22.3.3. Adding an IPv6 address to an interface</h3></div></div></div>
<p>
If there are any router advertisers on a network there is a chance
that the interfaces on that network already received an IPv6 address
when the IPv6 kernel support was loaded. If this is not the case
an IPv6 address can be added to an interface using the <span class="command"><strong>ip</strong></span>
utility. Suppose we want to add the address <span class="quote">“<span class="quote">fec0:0:0:bebe::1</span>”</span>
with a prefix length of 64 (meaning <span class="quote">“<span class="quote">fec0:0:0:bebe</span>”</span> is the 
prefix). This can be done with the following command syntax:
</p>
<pre class="screen">
# <span class="command"><strong>ip -6 addr add &lt;ip6addr&gt;/&lt;prefixlen&gt; dev &lt;device&gt;</strong></span>
</pre>
<p>
For example:
</p>
<pre class="screen">
# <span class="command"><strong>ip -6 addr add fec0:0:0:bebe::1/64 dev eth0</strong></span>
</pre>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="idm961615700"></a>22.4. Wireless interfaces</h2></div></div></div>
<p>
Wireless interfaces usually require some additional configuration, like
setting the ESSID, WEP keys and the wireless mode. Interface settings that
are specific to wireless interfaces can be set in the 
<html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.wireless.conf</code></html:span> file. The
<span class="command"><strong>/etc/rc.d/rc.wireless</strong></span> script configures wireless
interfaces based on descriptions from
<html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.wireless.conf</code></html:span>. In
<html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">rc.wireless.conf</code></html:span> settings are made per interface
MAC address. By default this file has a section that matches any interface:
</p>
<pre class="screen">
## NOTE : Comment out the following five lines to activate the samples below ...
## --------- START SECTION TO REMOVE -----------
## Pick up any Access Point, should work on most 802.11 cards
*)
    INFO="Any ESSID"
    ESSID="any"
    ;;
## ---------- END SECTION TO REMOVE ------------
</pre>
<p>
It is generally a good idea to remove this section to make per-card
settings. If you are lazy and only have one wireless card, you can leave
this section in and add any configuration parameters you need. Since this
section matches any wireless interface the wireless card you have will
be matched and configured. You can now add a sections for your wireless
interfaces. Each section has the following format:
</p>
<pre class="screen">
&lt;MAC address&gt;)
    &lt;settings&gt;
;;
</pre>
<p>
You can find the MAC address of an interface by looking at the 
<span class="command"><strong>ifconfig</strong></span> output for the interface. For example,
if a wireless card has the <span class="emphasis"><em>eth1</em></span> interface name,
you can find the MAC address the following way:
</p>
<pre class="screen">
# <span class="command"><strong>ifconfig eth1</strong></span>
eth1      Link encap:Ethernet  HWaddr <span class="emphasis"><em>00:01:F4:EC:A5:32</em></span>
          inet addr:192.168.2.2  Bcast:192.168.2.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4 errors:1 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 b)  TX bytes:504 (504.0 b)
          Interrupt:5 Base address:0x100
</pre>
<p>
The hexadecimal address that is printed after <span class="emphasis"><em>HWaddr</em></span>
is the MAC address, in this case <span class="emphasis"><em>00:01:F4:EC:A5:32</em></span>.
When you have found the MAC address of the interface you can add a section
for the device to <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.wireless.conf</code></html:span>. For
example:
</p>
<pre class="screen">
00:01:F4:EC:A5:32)
    INFO="Cabletron Roamabout WLAN NIC"
    ESSID="home"
    CHANNEL="8"
    MODE="Managed"
    KEY="1234-5678-AB"
    ;;
</pre>
<p>
This will set the interface with MAC address
<span class="emphasis"><em>00:01:F4:EC:A5:32</em></span> to use the ESSID
<span class="emphasis"><em>home</em></span>, work in <span class="emphasis"><em>Managed</em></span> mode on
channel <span class="emphasis"><em>8</em></span>. The key used for WEP encryption is
<span class="emphasis"><em>1234-5678-AB</em></span>. There are many other parameters
that can be set. For an overview of all parameters, refer to the last
example in <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">rc.wireless.conf</code></html:span>.
</p>
<p>
After configuring a wireless interface, you can activate the changes
by executing the network initialization script
<span class="command"><strong>/etc/rc.d/rc.inet1</strong></span>. You can see the current wireless
settings with the <span class="command"><strong>iwconfig</strong></span> command:
</p>
<pre class="screen">
eth1      IEEE 802.11-DS  ESSID:"home"  Nickname:"HERMES I"
          Mode:Managed  Frequency:2.447 GHz  Access Point: 02:20:6B:75:0C:56   
          Bit Rate:2 Mb/s   Tx-Power=15 dBm   Sensitivity:1/3  
          Retry limit:4   RTS thr:off   Fragment thr:off
          Encryption key:1234-5678-AB
          Power Management:off
          Link Quality=0/92  Signal level=134/153  Noise level=134/153
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:27  Invalid misc:0   Missed beacon:0
</pre>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="idm961615636"></a>22.5. Resolving</h2></div></div></div>
<h3>
<a name="idm961606508"></a>Hostname</h3>
<p>
Each computer on the internet has a hostname. If you do not have a
hostname that is resolvable with DNS, it is still a good idea to
configure your hostname, because some software uses it. You can
configure the hostname in <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/HOSTNAME</code></html:span>.
A single line with the hostname of the machine will suffice. Normally
a hostname has the following form: host.domain.tld, for example
darkstar.slackfans.org. Be aware that the hostname has to be resolvable,
meaning that GNU/Linux should be able to convert the hostname to an IP
address. You can make sure the hostname is resolvable by adding it
to <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/hosts</code></html:span>. Read the following section for
more information about this file.
</p>
<h3>
<a name="idm961605132"></a>/etc/hosts</h3>
<p>
<html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/hosts</code></html:span> is a table of IP addresses with
associated hostnames. This file can be used to name computers
in a small network. Let's look at an example of the
<html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/hosts</code></html:span> file:
</p>
<pre class="screen">
127.0.0.1               localhost
192.168.1.1             tazzy.slackfans.org tazzy
192.168.1.169           flux.slackfans.org
</pre>
<p>
The <span class="emphasis"><em>localhost</em></span> line should always be present. It assigns the name <span class="emphasis"><em>localhost</em></span>
to a special interface, the loopback. In this example the names
<span class="emphasis"><em>tazzy.slackfans.org</em></span> and <span class="emphasis"><em>tazzy</em></span> are assigned to the IP address 192.168.1.1,
and the name <span class="emphasis"><em>flux.slackfans.org</em></span> is assigned to the IP address 192.168.1.169.
On the system with this file both computers are available via the
mentioned hostnames.
</p>
<p>
It is also possible to add IPv6 addresses, which will be used if your system
is configured for IPv6. This is an example of a <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/hosts</code></html:span>
file with IPv4 and IPv6 entries: 
</p>
<pre class="screen">
# IPv4 entries
127.0.0.1               localhost
192.168.1.1             tazzy.slackfans.org tazzy
192.168.1.169           gideon.slackfans.org

# IPv6 entries
::1			localhost
fec0:0:0:bebe::2	flux.slackfans.org	
</pre>
<p>
Please note that <span class="quote">“<span class="quote">::1</span>”</span> is the default IPv6 loopback.
</p>
<h3>
<a name="idm961601164"></a>/etc/resolv.conf</h3>
<p>
The <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/resolv.conf</code></html:span> file is used to specify
which nameservers the system should use. A nameserver converts hostnames
to IP addresses. Your provider should have given you at least two name
name server addresses (DNS servers). You can add these nameservers
to <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/resolv.conf</code></html:span> by adding the line
<span class="emphasis"><em>nameserver ipaddress</em></span> for each nameserver. For example: 
</p>
<pre class="screen">
nameserver 192.168.1.1
nameserver 192.168.1.169
</pre>
<p>
You can check wether the hostnames are tranlated correctly or not with
the <span class="command"><strong>host hostname</strong></span> command. Swap <span class="emphasis"><em>hostname</em></span> with an
existing hostname, for example the website of your internet service
provider.
</p>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="idm961606724"></a>22.6. IPv4 Forwarding</h2></div></div></div>
<p>
IPv4 forwarding connects two or more networks by sending packets
which arrive on one interface to another interface. This makes it
possible to let a GNU/Linux machine act as a router. For example, you
can connect multiple networks, or your home network with the internet.
Let's have a look at an example:
</p>
<div class="figure">
<a name="router"></a><p class="title"><b>Figure 22.2. Router example</b></p>
<div class="figure-contents"><div class="mediaobject"><img src="../../images/router.png" alt="Router example"></div></div>
</div>
<br class="figure-break"><p>
In the example there are two networks, 192.168.1.0 and 192.168.2.0. Three
hosts are connected to both network. One of these hosts is connected to
both networks with interfaces. The interface on the 192.168.1.0 network
has IP address 192.168.1.3, the interface on the 192.168.2.0 network has
IP address 192.168.2.3. If the host acts as a router between both networks
it forwards packets from the 192.168.1.0 network to the 192.168.2.0 network
and vise versa. Routing of normal IPv4 TCP/IP packages can be enabled
by enabling IPv4 forwarding.
</p>
<p>
IPv4 forwarding
can be enabled or disabled under Slackware Linux by changing the
executable bit of the <html:span xmlns:html="http://www.w3.org/1999/xhtml" class="filename"><code class="filename">/etc/rc.d/rc.ip_forward</code></html:span>
file. If the executable bit is set on this file, IP forwarding will
be enabled during the system boot, otherwise it will not. You can
check whether the executable bit is enabled with <span class="command"><strong>ls -l</strong></span>
(a description of the <span class="command"><strong>ls</strong></span> command can be found in
<a class="xref" href="chap-filesystem.html#chap-filesystems-analyzing-listing" title="8.2.1. Listing files">Section 8.2.1, “Listing files”</a>). 
</p>
<p>
It is also possible to enable IPv4 forwarding on a running system
with the following command (0 disables forwarding, 1 enables forwarding):
</p>
<pre class="screen">
# <span class="command"><strong>echo 0 &gt; /proc/sys/net/ipv4/ip_forward</strong></span>
</pre>
<p>
Be cautious! By default there are no active packet filters. This means
that anyone can access other networks. Traffic can be filtered
and logged with the iptables kernel packet filter. Iptables can
be administrated through the <span class="command"><strong>iptables</strong></span> command.
NAT (Network Address Translation) is also a subset of iptables,
and can be controlled and enabled through the <span class="command"><strong>iptables</strong></span>
command. NAT makes it possible to <span class="quote">“<span class="quote">hide</span>”</span> a network behind one
IP address. This allows you to use the internet on a complete network
with only one IP address.
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